1. Field of the Invention
The present invention relates to implantable medical devices and, more specifically, to medical devices that provide data to clinicians for analysis.
2. Description of the Related Art
There are numerous types of implantable medical devices (IMDs) that are available for gathering information and/or delivering therapy. In the cardiac arena, pacemakers (implantable pulse generators (IPGs)) and implantable cardioverter/defibrillators (ICDs) are the most common, and it should be appreciated that ICDs often also include pacing capabilities. The assignee of the present invention also makes an implantable loop recorder (ILM) that does not deliver any electrical therapy, but does monitor and record various cardiac signals. The IMDs will have a memory unit that stores certain types of information and transmits that information to an external device through a telemetry session for use and analysis.
When an IMD has pacing capabilities, that device is programmed to operate in a given mode. For example, a commonly used mode is referred to as DDD/R. This means that the IMD can sense and pace in multiple chambers of the heart (typically, the right atrium and right ventricle) and is rate responsive. The DDD/R mode is very comprehensive and provides for almost complete control of cardiac timing. After each atrial event (either intrinsic or paced), an AV interval (AVI) is started. At the end of the AVI, the device will deliver a ventricular pacing pulse, unless one has occurred intrinsically. However, since the DDD/R mode is comprehensive and attempts to provide a normalized cardiac cycle, the AVI is relatively short; thus, this almost always results in a ventricular pacing pulse occuring. In other words, a patient would need to have unusually fast intrinsic conduction time to have intrinsic ventricular depolarization when operating in a nominal DDD/R mode.
While the DDD/R mode is comprehensive and beneficial, there is a class of patients that have intact but “slow” AV conduction times. Thus, but for the setting of the AVI, the atrial event would naturally lead to the depolarization of the ventricles without having to provide ventricular pacing. There has been a recent recognition that allowing intrinsic conduction, even at longer intervals, is preferred to providing ventricular pacing. There are, of course, reasons why such pacing would be preferable or necessary. For example, a patient having complete heart block would be dependent upon ventricular pacing. Certain therapies, such as cardiac resynchronization therapy (CRT), pace both the left and right ventricles for heart failure patients.
As indicated, the recognition that ventricular pacing, particularly pacing in the right ventricular apex, is less desirable than permitting intrinsic conduction, is relatively recent. The assignee of the present invention has developed a mode that facilitates intrinsic conduction while providing ventricular pacing only when necessary. One commercial embodiment of this mode is referred to as the Managed Ventricular Pacing™ mode or the MVP™ mode. In this mode, a full cardiac cycle is permitted to elapse without providing ventricular pacing. Thus, for a given cycle, this affords the maximum amount of time for intrinsic conduction to occur. If no ventricular activity occurs in a given cycle, pacing is provided in the subsequent cycle; and, if this occurs too frequently (e.g., 2 out 4 cycles), then ventricular pacing in a standard mode (e.g., DDD/R) is provided for a period of time. Periodic conduction checks are performed to determine if intrinsic conduction has returned, as such block is often transient in patients. This discussion is meant to be exemplary and illustrative and in no way limiting of the MVP™ mode or other modes.
There are other modes that seek to extend or maximize duration during which intrinsic conduction is permitted. Some of these modes do not permit a complete cardiac cycle to transpire without ventricular activity, but provide the longest permissible AV interval such that a delivered ventricular pace will not adversely interfere with the subsequent atrial event (from a timing perspective). As used herein, these modes are collectively referred to as atrial-based pacing mode. In short, atrial-based pacing modes utilize A-A intervals to determine and/or control cardiac rate and seek to limit ventricular pacing by either tolerating a cycle devoid of ventricular activity or providing a relatively long AV interval as compared with standard DDD modes.
With atrial-based pacing, the atrial rate is known or knowable, but the actual AV delay is generally not known for a given cycle, as intrinsic conduction can occur at various times. The maximum variability occurs in modes that tolerate an absence of ventricular activity for a given cycle, as the actual AV delay may be equal to or any value less than the A-A interval, or there may not be a value for a given cycle. Comparing this to standard DDD/R timing, a typical AVI may be on the order of 150 ms, as an example. Thus, the AV delay could be 0-150 ms and, in practice, very short delays are likely to be PVCs, and thus, the range is 80-150 ms. In an atrial-based mode tolerating absent ventricular cycles, at a rate of 60 bpm, the AV delay may be 0-1000 ms. Thus, in atrial-based pacing modes the range of timing is greatly expanded.